This invention relates generally to electrochromic display devices utilizing the reversible color change of an electrochromic material due to electrochemical oxidation-reduction reaction, and more particularly, to electrochromic display devices wherein the electrochromic material comprises iron(III) hexacyanoferrate(II) salt.
With the advances being made in electronics technology, there is an increasing demand for display devices for use with portable information processing instruments, such as watches and electronic calculators. Conventional display devices used for these purposes usually include liquid crystal display devices. Although liquid crystal display devices have the advantages of fairly acceptable response speed and a somewhat reliable lifetime, they have the serious disadvantages of exhibiting dark display and reduced visibility when viewed at an angle (visual dependence). Liquid crystal devices also have visual and aesthetic limitations which restrict their use in some types of display devices. To provide a new display device which is free of the shortcomings of liquid crystal display devices, considerable research and development have been carried out involving electrochromic display devices utilizing the phenomenon of reversible color change which occurs in particular materials through electrochemical oxidation and reduction. Typical examples of such electrochemically color-developing materials (which are referred to hereinafter as "electrochromic materials") used in the prior art are viologen, which is composed of alkyl quaternary ammonium derivatives of .gamma.,.gamma.'-dipyridyl, and transition metal oxides, as exemplified by tungsten trioxide. The electrochromic material is applied as a layer or film onto the surface of an electrode to form what is commonly known in the art as a "display element" or "display electrode" and for convenience of description, the term "display electrode" is primarily used in the following description of the invention.
The present invention is discriminated from the prior art techniques in that it provides an electrochromic display device using a salt of iron(III) hexacyanoferrate(II) which has not heretofore been used as an electrochromic material.
In the case of viologen, which is a typical example of a conventional electrochromic material, when an electrolyte solution having viologen dissolved therein is subjected to electrolytic reduction, a viologen radical is formed as a colored deposit on the cathode surface according to the reaction of equation (1): ##STR1## wherein R is an alkyl group. Such systems, which involve a reaction resulting in the deposit of a coloring species from an electrolyte solution onto the electrode surface, have the following disadvantages: the quantity of charge must be precisely controlled per unit area of the electrode to provide a uniform tone, because the display color depends on the quantity of the deposit formed on the electrode surface, and they tend to deteriorate after repeated oxidation-reduction cycles, as the viologen itself lacks chemical stability.
Another conventional system using tungsten oxide, one of the transition metal oxides, as an electrochromic material, utilizes color development by tungsten bronze. The tungsten bronze is produced through a reaction caused by the simultaneous introduction of electrons from the electrode, and metal ions from the electrolyte solution, into the layer of tungsten oxide on the electrode surface, according to equation (2): ##EQU1##
In order that the reaction of equation (2) be electrochemically reversible, x should be equal to or less than 0.3 (x.ltoreq.0.3) in equation (2). If x is more than 0.3 (x&gt;0.3), the reaction becomes irreversible and thus incompatible with the display purpose. Display devices involving a non-stoichiometric reaction, such as required to effect the reversible production of tungsten bronze, are difficult to drive because the electrolysis must always be controlled so that x does not exceed the reversible range, and thus these prior art display device are not suitable for all commercial applications.